The invention relates to a hydraulic circuit for controlling a consumer and a method for controlling a hydraulic motor, wherein a like circuit is used.
Such circuits are employed in work tools wherein a positive or negative acceleration of an apparatus structure is to be accomplished in dependence on a pressure at the consumer. A typical application is, for example, the case of a rotating mechanism of a mobile excavator, the superstructure of which is moved through a rotating mechanism. The rotating mechanism is driven by means of a hydraulic motor supplied with pressure medium by a pump via a directional control valve.
In FIG. 1, which shall be referred to as early as now, a switching diagram for a like rotating mechanism drive is represented in a strongly simplified form. What is shown is a closed circuit, with the same principle being, of course, also applicable in the case of open circuits.
The closed circuit includes a hydrostatic pump 1 having the form of a variable displacement pump whereby a hydraulic motor 2 of the rotating mechanism is driven. Protection of the rotating mechanism drive is achieved with the aid of pressure control valves 4, 4xe2x80x2 which open towards the outlet side (low-pressure side ND) when a predetermined system pressure is exceeded in the meter-in side (high-pressure side HD). In this case, the pressure medium is conveyed past the hydraulic motor 2 from the supply directly into the outlet, so that pressure peaks in the high-pressure branch may be avoided. Upon reversing the conveying direction of the hydrostatic pump 1, the HD and ND branches are exchanged, so that for securing this condition the parallel pressure control valve 4, 4xe2x80x2 becomes effective.
Upon driving the hydraulic motor 1, the load pressure increases comparatively strongly because the stationary superstructure must initially be accelerated. Following the acceleration phase, i.e. after overcoming the start-up resistance, continued rotation of the superstructure may be achieved with comparatively lower forces, so that the load pressure drops to a lower level. When the super-structure is braked, a high pressure may correspondingly occur on the outlet side of the hydraulic motor. EP 0 603 421 A1 discloses an open circuit wherein fixedly set pressure control valves are used for limiting the various pressures to a maximum value. The pressure control valves may also be designed to be adjustable, so that a lower setting is predetermined through the force of a closing spring of the pressure control valve. This may be varied by a control pressure acting in the closing direction in addition to the force of the closing spring, so that an upper setting may be defined. Such a solution is disclosed in U.S. Pat. No. 5,460,000.
In EP 0 053 323 A1 a generic circuit is disclosed, wherein a control pressure acts in the opening direction of the pressure control valve, so that the adjustment of the outlet side pressure control valve may, for example, be diminished to a lower setting on a lower level.
In post-published patent application DE 19744337 to the same applicant, a pressure control valve is disclosed wherein the closing spring of a pilot control valve body is supported against a transformer piston which, in turn, may receive application of the pressure in the main stage of the pressure control valve for modification of the spring bias.
In practical use of the above described circuit variants it is found that upon decelerating a rotating mechanism, particularly where a low load or no load acts on the work equipment, abrupt stopping of the super-structure may occur. These abrupt loads may result in damage to the rotating mechanism or to the work equipment.
In contrast, the invention is based on the object of furnishing a hydraulic circuit for controlling a consumer and a method for controlling a hydraulic motor provided with a like circuit, wherein the operational safety of the work tool is improved while incurring minimum expense in terms of device technology.
This object is attained with respect to the hydraulic circuit and with respect to the method for controlling a hydraulic motor.
Owing to the measure of controlling the pilot controlled pressure control valve in the outlet or supply line, respectively, in dependence on a pilot load pressure reflecting the load, for example the mass moment of inertia of the work tool, the braking pressure controlled through the pressure control valve may be selected in accordance with this load.
I.e., in the case of a low load of the work tool, a correspondingly low braking pressure is adjusted, so that the work tool is decelerated with a comparatively low braking force. In cases of high loads, a correspondingly higher pressure is adjusted in the outlet line, so that the braking performance is always optimally adapted to the operating conditions.
In mobile work tools having a rotating mechanism drive it is an option to select control of the pressure control valves in dependence on the mass moment of inertia of the superstructure. The pilot load pressure corresponding to the mass moment of inertia may, for example, be tapped at the lifting cylinders located between the superstructure and the boom, so that a comparatively low expense in terms of circuit technology is required for realising the circuit according to the invention.
This load pressure e.g. corresponding to the moment of inertia of the superstructure is reported directly to a pilot control valve body of the pressure control valve or to a transformer piston, so that the closing force acting on the pilot control valve body is adjustable in accordance with the load of the work tool. Herein the minimum pilot load pressure is predetermined through the force of the closing spring, while the maximum pilot load pressure is determined by the force of the closing spring and by the detected load pressure.
In principle it is moreover conceivable to electrically perform control of the pressure control valve while electrically controlling the pressure control valve in accordance with the load pressure (moment of inertia).
The hydraulic circuit is preferably equipped with a shuttle valve having the pilot control line connected to its output, and at the inputs of which the load pressure of the first named consumer of the work tool and the load pressure corresponding to the load of the work tool of another consumer are reported.
In a preferred variant of the hydraulic circuit, the pilot load pressure acts on a transformer piston on which the closing spring for the pilot control valve body is supported.
As an alternative, the pilot control valve body may moreover receive direct application of the pilot load pressure.
It is particularly advantageous if the pressure control valves are designed as anti-cavitation valves.
The ratio of the diameters of the transformer piston and of the valve seat of the pilot control valve body preferably is 1.1:1.